This invention relates to color television receiving apparatus and, more particularly, to improvements in such apparatus whereby the color characteristics of a reproduced color video picture are controlled in accordance with a transmitted reference signal, such automatic control also being subjected to manual adjustment.
In a conventional NTSC composite color television signal, monochrome information, that is, the brightness information of the video picture, is transmitted by the luminance signal and the color information is transmitted by the chrominance signal. This chrominance information is amplitude modulated onto a subcarrier, and different color information components are modulated onto different phases of the subcarrier. The composite color video signal also includes periodic horizontal synchronizing signals and color burst signals which are used at the television receiver to control a local oscillator such that its frequency and phase are synchronized with that of the burst signal. For this purpose, the color burst signal is formed of approximately eight cycles of the color subcarrier which is superimposed onto the pedestal level during the horizontal synchronizing interval of the video signal. The composite video signal also includes vertical synchronizing information which is present during a number of horizontal line intervals that form the vertical retrace interval.
As aforementioned, the purpose of the color burst signal is to synchronize a local oscillator in a color television receiver so that the local oscillator output can be used to demodulate the received chrominance component with sufficient fidelity such that the reproduced color video picture is an accurate image of the original transmitted picture. However, a transmitted composite color video signal may be subjected to various phase and/or amplitude distortions. Such distortions may be derived directly at the transmitting site or may be attributed to the signal transmission path. Although the purpose of the burst signal is to permit the local oscillator at a color television receiver to match or track such distortions, there is the possibility that the burst signal and modulated color subcarriers may be subjected to different distortions. That is, since the burst signal is provided on the pedestal level and the color-modulated subcarrier is provided on quite different levels, the amplitude and phase distortion at the pedestal level might differ from such distortion at the other level. Consequently, even though this distortion in the burst level can be detected and corrected, such correction may not fully account for the distortions in the subcarrier at the other level.
One proposal to minimize distortion in the subcarrier which may not be fully corrected by synchronizing the local oscillator at the television receiver to the received burst signal is to provide a further reference signal in the transmitted composite color video signal. This reference signal is known as the vertical interval reference signal, or VIR signal, and is formed of a reference subcarrier, equal in frequency and phase to the burst signal, transmitted during a selected portion of the vertical retrace interval. It has been suggested that this VIR signal can be transmitted during the nineteenth or twentieth horizontal line interval which, as is known, is included in the vertical retrace interval. Essentially, this subcarrier reference signal differs from the burst signal in that is exists for a longer duration and is superimposed on a level other than the pedestal level.
It is expected that, when a transmitted video signal having the VIR signal is received, the reference information provided by the VIR signal can be used to control the color television receiving apparatus such that the reproduced color picture exhibits optimum color characteristics. That is, this VIR signal can be used to control the phase of the local oscillator which, as is known, determines the hue or tint of the color picture. Also, the information provided by this VIR signal can be used to control the level of the received chrominance component which, as is known, controls the saturation of the reproduced color picture. Therefore, by automatically controlling the local oscillator phase and chrominance component level in accordance with optimum standards, such as the NTSC standard, as represented by the VIR signal, a correspondingly optimum color video picture can be reproduced.
Although an optimum color picture can be reproduced so as to conform with predetermined standards, it is quite possible that a television viewer might have certain preferences which differ from the standard characteristics. That is, the particular hue and saturation characteristics of the reproduced optimum picture might not conform to the viewer's particular preferences. However, in proposed color television receiving apparatus which utilizes the VIR signal for controlling the color characteristics in the reproduced picture, no provision is made to enable a viewer to selectively adjust the hue or saturation so that the picture differs from the optimum picture. Another disadvantage of some proposed VIR-responsive color television receiving apparatus is that the circuitry which is provided specifically for use with the VIR signal is not used in the event that the transmitted composite color video signal does not include the VIR signal. This means that the hue and saturation control circuitry either is inhibited or must remain idle during broadcasts which do not include the VIR signal. Consequently, the preferred color control which is achieved by the circuitry used with the VIR signal is not utilized advantageously in the absence of such a reference signal.